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  1. #1
    Join Date
    Aug 2019
    Sydney, Australia

    Default CNC to concrete hole saw adapter

    Saturday, someone walked into the man cave and said they wanted me to machine an adapter to fit a hole saw. The current tool holder is from some weird sort of CNC a little tapered and threaded thingy:


    This thingy has a 14tpi thread, 30/60 taper where it goes into the CNC, and water flowing through the middle (6mm hole). They want to put a diamond or CBN hole saw on the end the sample had an M14 thread in its flange:


    There was not a lot of metal on the end of the thingy, especially with the 6mm hole, but it is only cutting marble, so I was willing to risk it. The sordid tale:

    1. My Hercus has limited change gears and is imperial, so the M14 would need another lathe. Went to a neighbour to use his Hafco (AL330?).
    2. Changed the gears to allow 2mm pitch. This lathe has a damaged brake, but having a cam-lock chuck means I can thread in reverse!
      (cut left to right, feeding down on the back of the chuck)
    3. Started with some scrap rod. Turned down to 13.8mm. Fitted an internal threading bar (11 IR AG60), set compound 30 off cross-slide, moved tool behind the job to about 3cm from end of rod.
    4. Checked chuck rotation (up on the red lever is back of chuck feeding down. Must remember that!)
    5. Checked carriage feed. Oops, feeding toward chuck. Off, grab chuck, manually brake.
      Wonder if I should be wearing gloves. Turn feed selector the other way.
    6. Move tool to the surface, zeroed cross slide. Turn compound slide back (toward me and the job) by a smidge probably .05mm, but this isn't my lathe and I can never read the graduations on these things.
    7. Engage the lever. A nice little shiny spiral peels off, the tool safely moves through empty air, and I turn the power/feed off in a relaxed manner.
    8. Wind cross slide in a turn. Lever down, then back up to turn off. Gets close to my starting point.
      I hand-turn the chuck a few turns, then back again to take up the slack/hysteresis/backlash.
      Cross slide back to zero. Compound back to me by another smidge. Lever up.
    9. A bit of chatter at the start (the flex in the boring bar is doing the initial plunge in my thread),
      then another shiny spiral. This might just work?
      Another 10 or 12 passes, and an acceptable M14 thread!IMG_0573-rod.jpg
    10. Now for the real thing. Mount/centre/dial-in the thingy. Turn down to 16.27mm (diameter of a small un-threaded step inside the hole saw flange), then down to 13.7ish. Plunge thread in as close to step as I can get, 10 or 15 passes, get down to M14-ish size.
    11. Now, trial fit of the hole saw. Feels good going on, but is 2 or 3mm away from screwing all the way on. Decide it must be a tapered thread, do a few more passes. Trial fit again, no better. Go cross-eyed double-checking my step height. Decide it must be the shape of the step needs to have a nice chamfer on the edge.
    12. Now, the problem. Job mounted in chuck, spindle rotationally locked to feed screw for threading, but I need to change cutting tools and set carriage free so I can shorten this damn locating step I left in. Thread-chasing dial is just below 5. OK. Must remember that.
    13. Disengage saddle. Change tool. Try to machine 45 on RHS of step. Refit threading tool. Fiddle around with saddle to try and get it to engage just below 5. Do some more thread cutting a few extra mm long.
    14. Trial fit hole saw again. Not much better. Hmmm.
      Neighbour wants to go home and self-isolate from a virus. OK, Monday's problem

    (will finish the story tomorrow)

    Fast-forward to Monday. I said this would be ready by lunchtime, and my neighbour gets in around 10. Rush job!

    15. Surely I took enough off the front of the step? Lathe owner (a toolmaker) looks at hole saw. His eyesight is better than mine. He thinks the un-threaded bit is flat, not chamfered. Hmmm. OK. I'll hack a bit more off the step. Swap tools, disengage carriage feed, get it down to about 1/8"

    16. Swap tools back, start plunging the thread start closer to the step, take a few passes, notice that the material is coming off the whole length of the thread, and not just the start. WHAT?

    17. Don't have time to stop, find an eye loupe, and inspect closely, so I do a few more passes, winding the cut back out after the initial plunge. A few more trial fits and it is getting close. However, on one of the forward feeds (to get tool back to starting plunge point) I stuff up and drive the tool into the flange on the job. Grrr. Re-align, a few more threading passes.

    18. Trial fit, less than 1mm from the flange, but the thread is feeling terrible. Finally stop and think. Look closely at the flange. Where the face forms the new step, there is a radius. Look closely at the thread. It now has two peaks. Something has moved. GRRR. The fourth thread I have ever cut on a lathe is a bit of a failure IMG_0573-bad.jpg

    19. Back to the DCMT tool, rotate a bit on the tool post, and remove that radius with an undercut. Success the saw now fits. I just hope they don't tighten it too much.

    20. Using all that I have learnt, the second one should be fast and accurate, right?
    Mount, centre, turn down to 16.3, leave a modest step, down to 13.7. Many passes with the threading tool. Trial fit. Feels OK, but still a few mm away from the flange. What? This was meant to be the good one!

    21. Swap tools, shorten step a bit more, back to threading tool. Neighbour wants to go and look at a tool grinder. I say it should only be 3 minutes. I plunge and start another threading cut, and clunk. What?

    22. Tip broken. Turn around to the next free one (best thing about TCT). He helps me try to re-align in existing cut thread, but job now off centre. Sigh. He leaves me his spare key.

    23. Re-mount job in chuck, white paper underneath so I can see tool position in thread, remove broken shrapnel from thread, hand-turn a few threading passes with thread cutting compound, to get something close to an acceptable thread. 1:30PM, two useable thingies, but one M14 is 13.5mm and one is 13.2


    So, my learnings;

    A) Swapping tools and operations is risky. Even with the threading counter, and a good QCTP, things don't line up perfectly

    B) Don't ever promise an experimental/learning job in a hurry.

    (as it is, he hasn't come to pickup the job yet, so I needn't have rushed)

    C) Sometimes, it would be easier to just buy an M14 threading die.

    (but not for the $20 I said my time would cost him)

    ...damn extra thumbnails that I deleted...
    Attached Images Attached Images
    Last edited by nigelpearson; 24th Mar 2020 at 08:44 AM. Reason: finish the story

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